How do I pass each element of a slice as a separate argument to a variadic C function?

Question

I'm building a Redis Module in Rust. I've found some good examples, but I'm stuck when dealing with interfacing a C function that is supposed to accept variadic arguments.

The Redis Module C SDK has a function called RedisModule_Call which accepts a few specific arguments then n arguments that represent a Redis command. From the Redis Module SDK documentation (in C):

RedisModuleCallReply *reply;
reply = RedisModule_Call(ctx,"INCR","sc",argv[1],"10");

RedisModule_Call's first three arguments are specific, but the rest represent Redis commands that could easily have hundreds of arguments.

In Rust, I'm following the patterns in Redis-Cell which is a Redis module implemented (successfully) in Rust. The module is fantastic but has a very limited way of dealing with this particular problem. Effectively, it accepts up to three arguments in a somewhat brute force way:

pub fn call(&self, command: &str, args: &[&str]) -> Result<Reply, CellError> {
   // ... code ... 
   let raw_reply = match args.len() {
        1 => raw::call1::call(/* ... */),
        2 => raw::call2::call(/* ... */),
        // ...

These call1 and call2 functions are practically just stubs that handle the different argument lengths:

pub mod call2 {
    use redis::raw;

    pub fn call(
        ctx: *mut raw::RedisModuleCtx,
        cmdname: *const u8,
        fmt: *const u8,
        arg0: *mut raw::RedisModuleString,
        arg1: *mut raw::RedisModuleString,
    ) -> *mut raw::RedisModuleCallReply {
        unsafe { RedisModule_Call(ctx, cmdname, fmt, arg0, arg1) }
    }

    #[allow(improper_ctypes)]
    extern "C" {
        pub static RedisModule_Call: extern "C" fn(
            ctx: *mut raw::RedisModuleCtx,
            cmdname: *const u8,
            fmt: *const u8,
            arg0: *mut raw::RedisModuleString,
            arg1: *mut raw::RedisModuleString,
        ) -> *mut raw::RedisModuleCallReply;
    }
}

I need to be able to pass in n arguments, with n being determined at run time, so this method of hard coding isn't practical. I know Rust has limited support for variadic functions and I've been doing some reading about RFC 2137, but I'm not sure this applies.

I'm looking for a way to apply an argument vector to the end of RedisModule_Call or something like a spread syntax for the arguments. I'm relatively new to Rust but I've searched and searched and I can't seem to find any way to scratch this itch in Rust.

To clarify - I can pass arguments into RedisModule_Call (which is variadic) no problem, but I can't find a syntactical way to pass in a variable number of arguments in Rust into the C function. What I'm trying to accomplish is something like this:

impl Redis {
    pub fn call(&self, command: &str, args: &[&str]) -> Result<Reply, CellError> {
        /* ... */

       unsafe { RedisModule_Call(ctx, cmdname, fmt, ...args) }
       /* ... */ 

Where ...args is some sort of black magic to that would allow args to represent 1 argument or 100 which would be the equivalent of RedisModule_Call(ctx, cmdname, fmt, args[0], args[1] /* ... and so on */).

Answer 1

You don't, at least not yet, and I'd wager probably never.

To be able to do this, you'd need two key abilities, both of which are outside of your control:

1. Redis needs to provide a function that accepts a va_list argument, not just a ....

   It's strange that Redis doesn't already provide such a function, but perhaps this is a sign that other people implementing modules avoid the problem entirely.

2. Rust needs to provide a way to construct the va_list argument.

   While it looks like RFC 2137 will introduce a VaList type, the proposed API does not provide a way to create one or set values in it.

Note that you can't do what you want, even in C (at least not easily or portably).

---

What can you do instead? Assuming that you are implementing the code that consumes the variadic arguments, you can remove the variation from your call. A collection of items in C is just a pointer and a length, so pass that instead:

extern "C" {
    fn call(n_args: i32, ...);
}

fn x(args: &[i32]) {
    unsafe { call(2, args.len(), args.as_ptr()) };
}

If you didn't have control of what reads the code on the other side, one possible (read: terrible) idea is to pattern match on some "large enough" subset of the slice and dispatch off to the variadic function:

extern "C" {
    fn call(n_args: i32, ...);
}

fn x(args: &[i32]) {
    unsafe {
        match args {
            [] => call(0),
            [a] => call(1, a),
            [a, b] => call(2, a, b),
            _ => panic!("Didn't implement this yet"),
        }
    }
}

See also:

• How to wrap a call to a FFI function that uses VarArgs in Rust?
• Populating a va_list
• Forward an invocation of a variadic function in C